Just on a purely personal note, if you want some money to go to the developers then I wouldn't use CDKeys as they don't deal directly with suppliers/distributers.
Why spend more than its needed? Does they deserve more money? You really think that?
If there is option to get this DLC regardless of source but cheaper, then anyone should do it (long as its legit - not pirated)...
As an idea of "supporting" developers seems not working as expected, thats the sad fact. Frontier by way they acts, shows that they dont care about Elite that much, so preety safe to say is that even with unlimited budget, they still would be doing anything else that support that game properly. So yeah, anyone who thinks that if they spend more on game, that it would help with anything with that, they are simply delusional.
Only times when Elite was "properly" taken care of, it was only during its first years, before Frontier started to "expand" and making other titles, as atleast back then before it happen, we had regular updates, and despite less content game had, there was less things to broken up atleast (those who did worked on it back then, atleast knew what they been doing - unlike now), whenever new update was relased.
Dont get me wrong, I did myself spend a lot into elite... First by mecenary edition (pre-relase) then both DLC (Horizons + oddy) each by 40 pounds, and probly 3 or 4 times as much worth of cosmeticals for each, by almost 10 years. I dont think I ever spend that much into any other game, not even by close margin. Does it was worth it? I say yea, since I do have thousands of hours done... but current state of title is leaving lot to be desired.
Im not mad about lack of new features more than I am mad because it lacks of proper care. Things getting broken each update, and by things get broken over and over again... and worst is who knows when it will be ever fixed! Waiting 8-9 months or even years in some cases, to get something fixed is not right, and honestly it sets me off.